Hi I Was Diagnosed With B12 Deficiency About 2 Years

Hi I was diagnosed with b12 deficiency about 2 years ago after having the symptoms for 25 years. I had the initial loading injections and felt better for about 4-6 weeks max I went back as my symptoms were back but was told it can't possibly be b12 now. I've been back a number of times with the same symptoms only to be told I should take antidepressants I admit I was/do feel low but I don't see this as the answer.
My symptoms include:
Excessive fatigue
Hot flushes/night sweats
Joint pain
Leg and ankle pain to the point I could barely walk
Migraines
Hair loss
Brittle nails
Stomach pain
Muscle weakness
Irregular heartbeat (had an ecg)
Mouth ulcers
Brain fog
Lack of concentration

I've seen numerous doctors but don't seem to be getting anywhere and I'm sure I shouldn't feel like this at 38 I feel shocking most days.
Can you please advise on what I can do to treat my symptoms or how to get my doctors to take it seriously as I've been made to feel I'm faking it for years.